RFC 4842 SONET/SDH Circuit Emulation April 2007
The basic CEP packet appears in Figure 1.
+-----------------------------------+
| PSN and Multiplexing Layer |
| Headers |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| RTP Header |
| (RFC 3550) |
+-----------------------------------+
| |
| |
| SONET/SDH |
| Fragment |
| |
| |
+-----------------------------------+
Figure 1: Basic CEP Packet
5.1. SONET/SDH Fragment
The SONET/SDH fragments MUST be byte aligned with the SONET/SDH SPE
or VT. The first bit received from each byte of the SONET/SDH MUST
be the Most Significant Bit of each byte in the SONET/SDH fragment.
SONET/SDH bytes are placed into the SONET/SDH fragment in the same
order in which they are received.
SONET/SDH optical interfaces use binary coding and therefore are
scrambled prior to transmission to ensure an adequate number of
transitions. For clarity, this scrambling is referred to as physical
layer scrambling/descrambling.
In addition, many payload formats (such as for Asynchronous Transfer
Mode (ATM) and High-Level Data Link Control (HDLC)) include an
additional layer of scrambling to provide protection against
transition density violations within the SPEs. This function is
referred to as payload scrambling/unscrambling.
CEP assumes that physical layer scrambling/unscrambling occurs as
part of the SONET/SDH section/line termination Native Service
Processing (NSP) functions.
Malis, et al. Standards Track [Page 6]

RFC 4842 SONET/SDH Circuit Emulation April 2007
However, CEP makes no assumption about payload scrambling. The
SONET/SDH fragments MUST be constructed without knowledge or
processing of any incidental payload scrambling.
CEP implementations MUST include the H3 (or V3) byte in the SONET/SDH
fragment during negative pointer adjustment events, and MUST remove
the stuff byte from the SONET/SDH fragment during positive pointer
adjustment events.
VT emulation implementations MUST NOT carry the VT pointer bytes V1,
V2, V3, and V4 as part of the CEP payload. The only exception is the
carriage of V3 during negative pointer adjustment as described above.
All CEP SPE implementations MUST support a packet size of 783 bytes
and MAY support other packet sizes.
VT emulation implementations MUST support payload size of full VT
super-frame, and MAY support 1/2 and 1/4 VT super-frame payload
sizes.
Table 1 below describes single super-frame payload size per VT type.
+-------------+--------------+
| VT type | Size (Bytes) |
+-------------+--------------+
| VT1.5/VC-11 | 104 |
| VT2/VC-12 | 140 |
| VT3 | 212 |
| VT6/VC-2 | 428 |
+-------------+--------------+
Table 1: VT Super-Frame Payload Sizes
OPTIONAL SONET/SDH Fragment formats have been defined to reduce the
bandwidth requirements of the emulated SONET/SDH circuits under
certain conditions. These OPTIONAL formats are included in
Section 11.
5.2. CEP Header
The CEP header supports both a basic and extended mode. The Basic
CEP header provides the minimum functionality necessary to accurately
emulate a SONET/SDH circuit over a PSN. Extended headers are
utilized for some of the OPTIONAL SONET/SDH fragment formats
described in Section 11.
Enhanced functionality and commonality with other real-time Internet
applications is provided by RTP encapsulation.
Malis, et al. Standards Track [Page 7]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The CEP header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|L|R|N|P|FRG|Length[0:5]| Sequence Number[0:15] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Structure Pointer[0:11]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: CEP Header Format
L bit: CEP-AIS. This bit MUST be set to 1 to signal to the
downstream PE that a failure condition has been detected on the
attachment circuit. Failure conditions leading to generation of
CEP-AIS and the mapping of CEP-AIS signal on the downstream
attachment circuit are described in Section 7.
R bit: CEP-RDI. This bit MUST be set to 1 to signal to the upstream
PE that a loss of packet synchronization has occurred. This bit
MUST be set to 0 once packet synchronization is acquired. See
Section 6.2 for details.
N and P bits: These bits are used to explicitly relay negative and
positive pointer adjustments events across the PSN. The use of N
and P bits is OPTIONAL. If not used, N and P bits MUST be set to
0. See Section 9 for details.
Table 2 describes the interpretation of N and P bits settings.
+---+---+-----------------------------+
| N | P | Interpretation |
+---+---+-----------------------------+
| 0 | 0 | No Pointer Adjustments |
| 0 | 1 | Positive Pointer Adjustment |
| 1 | 0 | Negative Pointer Adjustment |
| 1 | 1 | Loss of Pointer Alarm |
+---+---+-----------------------------+
Table 2: Interpretation of N and P Bits
FRG bits: FRG bits MUST be set to 0 by sender and ignored by
receiver.
SONET data is continuously fragmented into packets. The structure
pointer field designates the offset between the SONET SPE or VT
structure and the packet boundary.
Malis, et al. Standards Track [Page 8]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Length [0:5]: The Length field, if other than zero, indicates the
length of the CEP header, plus the length of the RTP header if
used, plus the length of the payload. The Length field MUST be
set if the length of CEP header plus RTP header if used, plus
payload is less than or equal to 64 bytes and MUST be set to 0
otherwise. In particular, if the payload is suppressed (e.g.,
DBA) the Length field MUST be set to the CEP header length plus
the RTP header length if used.
Sequence Number [0:15]: The packet sequence number MUST continuously
cycle from 0 to 0xFFFF. It is generated and processed in
accordance with the rules established in [RTP].
Structure Pointer [0:11]: The structure pointer MUST contain the
offset of the first byte of the SONET structure within the packet
payload. For SPE emulation, the structure pointer locates the J1
byte within the CEP packet. For VT emulation, the structure
pointer locates the V5 byte within the packet. The structure
pointer value ranges between 0 to 0xFFE, where 0 represents the
first byte after the CEP header. The structure pointer MUST be
set to 0xFFF if a packet does not carry the J1 (or V5) byte. An
arbitrary pointer change (New Data Flag (NDF) event) in the
attachment circuit changes the offset of the SONET structure
within the CEP packet and therefore changes the structure pointer
value.
Reserved field: The reserved field MUST be set to 0 by the sender
and ignored by receiver.
5.3. RTP Header
Usage of the RTP header is OPTIONAL. Although CEP MAY employ an RTP
header when explicit transfer of timing information is required, this
is purely a formal reuse of the header format. RTP mechanisms, such
as header extensions, contributing source (CSRC) list, padding, RTP
Control Protocol (RTCP), RTP header compression, Secure Realtime
Transport Protocol (SRTP), etc., are not applicable to pseudowires.
CEP uses the RTP Header as shown below.
Malis, et al. Standards Track [Page 9]

RFC 4842 SONET/SDH Circuit Emulation April 2007
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Source (SSRC) Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: RTP Header
V: Version. The Version field MUST be set to 2.
P: Padding. No padding is required. The P bit MUST be set to 0 by
sender and ignored by receiver.
X: Header extension. No extensions are defined. The X bit MUST be
set to 0 by sender and ignored by receiver.
CC: CSRC count. The CC field MUST be set to 0 by sender and ignored
by receiver.
M: Marker. The M bit MUST be set to 0 by sender and ignored by
receiver.
PT [0:6]: Payload type. A PT value SHOULD be allocated from the
range of dynamic values for each direction of the PW. The same PT
value MAY be reused both for direction and between different CEP
PWs.
Sequence Number [0:15]: The packet sequence number MUST continuously
cycle from 0 to 0xFFFF. It is generated and processed in
accordance with the rules established in [RTP]. The CEP receiver
MUST sequence packets according to the Sequence Number field of
the CEP header and MAY verify correct sequencing using RTP
Sequence Number field.
Timestamp [0:31]: Timestamp values are used in accordance with the
rules established in [RTP]. Frequency of the clock used for
generating timestamps MUST be 19.44 MHz based on a local
reference.
SSRC [0:31]: Synchronization source. The SSRC field MAY be used for
detection of misconnections.
Malis, et al. Standards Track [Page 10]

RFC 4842 SONET/SDH Circuit Emulation April 20075.4. PSN Encapsulation
This document defines the transport of CEP over MPLS PSNs. The
bottom label in the MPLS label stack MUST be used to de-multiplex
individual CEP channels. In keeping with the conventions used in
[PWE3-CONTROL], this de-multiplexing label is referred to as the PW
Label and the upper labels are referred to as Tunnel Labels. The CEP
header follows the generic PWE3 Control Word format specified in
[PWE3-MPLSCW] for use over an MPLS PSN.
Transport of CEP over other PSN technologies is out of scope of this
document.
6. CEP Operation
A CEP implementation MUST support a normal mode of operation and MAY
support additional bandwidth conserving modes as described in
Section 11. During normal operation, SONET/SDH payloads are
fragmented, prepended with the appropriate headers, and then
transmitted into the packet network.
6.1. CEP Packetizer and De-Packetizer
As with all adaptation functions, CEP has two distinct components:
adapting TDM SONET/SDH into a CEP packet stream, and converting the
CEP packet stream back into a TDM SONET/SDH. The first function is
referred to as CEP packetizer or sender and the second as CEP de-
packetizer or receiver. This terminology is illustrated below.
+------------+ +---------------+
| | | |
SONET --> | CEP | --> PSN --> | CEP | --> SONET
SDH | Packetizer | | De-Packetizer | SDH
| | | |
+------------+ +---------------+
(sender) (receiver)
Figure 4: CEP Terminology
The CEP de-packetizer requires a buffering mechanism to account for
delay variation in the CEP packet stream. This buffering mechanism
is generically referred to as the CEP jitter buffer.
During normal operation, the CEP packetizer receives a fixed-rate
byte stream from a SONET/SDH interface. When a packet worth of data
is received from a SONET/SDH channel, the necessary headers are
prepended to the SPE fragment and the resulting CEP packet is
transmitted into the packet network. Because all CEP packets
Malis, et al. Standards Track [Page 11]

RFC 4842 SONET/SDH Circuit Emulation April 2007
associated with a specific SONET/SDH channel have the same length,
the transmission of CEP packets for that channel SHOULD occur at
regular intervals.
At the far end of the packet network, the CEP de-packetizer receives
packets into a jitter buffer and then plays out the received byte
stream at a fixed rate onto the corresponding SONET/SDH channel. The
jitter buffer SHOULD be adjustable in length to account for varying
network delay behavior. On average, the receive packet rate from the
packet network should be balanced by the transmission rate onto the
SONET/SDH channel.
The CEP sequence numbers provide a mechanism to detect lost and/or
misordered packets. The sequence number in the CEP header MUST be
used when transmission of the RTP header is suppressed. The CEP de-
packetizer MUST detect lost or misordered packets. The CEP de-
packetizer SHOULD play out an all-ones pattern (AIS) in place of any
dropped packets. The CEP de-packetizer SHOULD re-order packets
received out of order. If the CEP de-packetizer does not support re-
ordering, it MUST drop misordered packets.
6.2. Packet Synchronization
A key component in declaring the state of a CEP service is whether or
not the CEP de-packetizer is in or out of packet synchronization.
The following paragraphs describe how that determination is made.
As packets are received from the PSN, they are placed into a jitter
buffer prior to play out on the SONET/SDH interface. If a CEP de-
packetizer supports re-ordering, any packet received before its play
out time will still be considered valid.
The determination of acquisition or loss of packet synchronization is
always made at SONET/SDH play out time. During SONET/SDH play out,
the CEP de-packetizer will play received CEP packets onto the SONET/
SDH interface. However, if the jitter buffer is empty or the packet
to be played out has not been received, the CEP de-packetizer will
play out an 'empty packet' composed of an all-ones AIS pattern onto
the SONET/SDH interface in place of the unavailable packet.
The acquisition of packet synchronization is based on the number of
sequential CEP packets that are played onto the SONET/SDH interface.
Loss of packet synchronization is based on the number of sequential
'empty' packets that are played onto the SONET/SDH interface.
Specific details of these two cases are described below.
Malis, et al. Standards Track [Page 12]

RFC 4842 SONET/SDH Circuit Emulation April 20076.2.1. Acquisition of Packet Synchronization
At startup, a CEP de-packetizer will be out of packet synchronization
by default. To declare packet synchronization at startup or after a
loss of packet synchronization, the CEP de-packetizer must play out a
configurable number of CEP packets with sequential sequence numbers
towards the SONET/SDH interface.
6.2.2. Loss of Packet Synchronization
Once a CEP de-packetizer is in packet synchronization state, it may
encounter a set of events that will cause it to lose packet
synchronization.
If the CEP de-packetizer encounters more than a configurable number
of sequential empty packets, the CEP de-packetizer MUST declare a
Loss of Packet Synchronization (LOPS) defect.
LOPS failure is declared after 2.5 +/- 0.5 seconds of LOPS defect,
and cleared after 10 seconds free of LOPS defect state. The circuit
is considered down as long as LOPS failure is declared.
7. SONET/SDH Maintenance Signals
This section describes the mapping of maintenance and alarm signals
between the SONET/SDH network and the CEP pseudowire. For clarity,
the mappings are split into two groups: SONET/SDH to PSN, and PSN to
SONET/SDH.
For coherent failure detection, isolation, monitoring, and
troubleshooting, SONET/SDH failure indications MUST be transferred
correctly over the CEP pseudowire, and CEP connection failures MUST
be mapped to SONET/SDH SPE/VT failure indications. Failure detection
capabilities and performance monitoring capabilities are dependent on
the NSP functionality, e.g., LTE, PTE, Tandem Connection Monitoring
[G.707], or Non-intrusive Monitoring (intermediate connection
monitoring).
7.1. SONET/SDH to PSN
The following sections describe the mapping of SONET/SDH Maintenance
Signals and Alarm conditions into CEP pseudowire indications.
7.1.1. CEP-AIS: AIS-P/V Indication
SONET/SDH Path outages are signaled by using maintenance alarms such
as Path AIS (AIS-P). AIS-P, in particular, indicates that the SONET/
SDH Path is not currently transmitting valid end-user data, and the
Malis, et al. Standards Track [Page 13]

RFC 4842 SONET/SDH Circuit Emulation April 2007
SPE contains all ones. Similarly, AIS-V indicates that the VT is not
currently transmitting valid end-user data, and the VT contains all
ones.
It should be noted that nearly every type of service-affecting
section or line defect would result in an AIS-P/V condition.
The mapping of SONET/SDH failures into the SPE/VT layer is considered
part of the NSP function and is based on the principles specified in
[GR253], [SONET], [G.707], [G.806], and [G.783]. For example, should
the SONET Section Layer detect a Loss of Signal (LOS) or Loss of
Frame (LOF) or Section Trace Mismatch (TIM) conditions, an AIS-L is
sent up to the Line Layer. If the Line Layer detects AIS-L or Loss
of Pointer (LOP), an AIS-P is sent to the Path Layer. If the Path is
terminated at the PE (i.e., a PTE) and the Path Layer detects AIS-P
or UNEQ-P or TIM-P or PLM-P an AIS-V is sent to the VT Layer.
The AIS-P/V indication is transferred to the CEP packetizer. During
AIS-P/V, CEP packets are generated as usual. The L bit in the CEP
header MUST be set to 1 to signal AIS-P/V explicitly through the
packet network. The N and P bits SHOULD be set to 1 to indicate loss
of pointer indication.
If DBA has been enabled for AIS-P/V, only the necessary headers and
optional padding are transmitted into the packet network. The Length
field MUST be set to the size of the CEP header plus the size of the
RTP header if used.
7.1.2. Unequipped Indication
Unequipped indication is used for bandwidth conserving modes defined
in Section 11 as a trigger for payload removal.
The declaration of the SPE/VT channel as Unequipped MUST conform to
[GR253], [SONET], [G.806], and [G.783]. Unequipped circuits do not
carry valid end-user data, but MAY be used for transporting valid
information in the SPE/VT overhead bytes. Supervisory Unequipped
signals and Tandem Connection transport are two such applications:
Supervisory Unequipped signal (called TEST-P in [SONET]) is used
to provide a test signal for pre-service testing or in-service
supervision of a path connection to a remote PTE from a PTE or an
intermediate non-terminating path network element. Both
Unequipped and Supervisory Unequipped signals carry Unequipped
Signal Label defined to be zero. To distinguish between
Unequipped and Supervisory Unequipped signals, [G.806] recommends
that the SPE/VT Trace bytes J1/J2 be set to a non-zero value in
Supervisory Unequipped signals.
Malis, et al. Standards Track [Page 14]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The SPE/VT overhead bytes N1/Z6 (SDH refers to Z6 as N2)
optionally transport Tandem Connection signals between
intermediate network elements. Unequipped signals transporting
Tandem Connection would have non-zero N1 or N2/Z6 bytes.
Therefore, the CEP packetizer MUST declare a circuit unequipped only
if the Signal Label, Trace (J1/J2), and Tandem Connection (N1/N2/Z6)
bytes all have zero value.
During SPE/VT Unequipped, the N and P bits MUST be interpreted as
usual. The SPE/VT MUST be transmitted into the packet network along
with the appropriate headers.
If DBA has been enabled for Unequipped SPE/VT, only the necessary
headers and optional padding are transmitted into the packet network.
The Length field MUST be set to the size of the CEP header plus the
size of the RTP header if used. The N and P bits MAY be used to
signal pointer adjustments as normal.
7.1.3. CEP-RDI: Remote Defect Indication
The CEP function MUST send CEP-RDI indication towards the packet
network during loss of packet synchronization by setting the R bit to
one in the CEP header. The CEP function SHOULD clear the R bit once
packet synchronization is restored.
7.2. PSN to SONET/SDH
The following sections describe the mapping of pseudowire indications
to SONET/SDH Maintenance Signals and Alarm conditions.
7.2.1. CEP-AIS: AIS-P/V Indication
There are several conditions in the packet network that cause the CEP
de-packetization function to play out an AIS-P/V indication towards a
SONET/SDH channel. The CEP de-packetizer MUST play out one packet's
worth of all ones for each packet received, and MUST set the SONET/
SDH Overhead to signal AIS-P/V as defined in [SONET], [GR253], and
[G.707].
The first of these is the receipt of CEP packets with the L bit set
to one indicating that the far end has detected an error leading to
declaration of AIS-P/V alarm. In addition to the play out of
AIS-P/V, the CEP de-packetizer SHOULD set the pointer value to all-
ones value.
Malis, et al. Standards Track [Page 15]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The second case that will cause a CEP de-packetization function to
play out an AIS-P/V indication onto a SONET/SDH channel is during
loss of packet synchronization.
The third case is the receipt of CEP packets with both the N and P
bits set to 1. This is an explicit indication of Loss of Pointer
LOP-P/V received at the far-end of the packet network. In addition
to play out of AIS-P/V, the CEP de-packetizer SHOULD set the pointer
value to all-ones value.
7.2.2. Unequipped Indication
There are several conditions in the packet network that cause the CEP
function to transmit Unequipped indications onto the SONET/SDH
channel.
The first, which is transparent to CEP, is the receipt of regular CEP
packets that happen to carry an SPE/VT containing the appropriate
Path overhead or VT overhead set to Unequipped. This case does not
require any special processing on the part of the CEP de-packetizer.
The second case is the receipt of CEP packets with the Length field
indicating that the payload was removed by DBA, while the L bit is
set to 0, indicating that the DBA was triggered by an Unequipped
indication and not by an AIS-P/V indication. The CEP de-packetizer
MUST use this information to transmit a packet of all zeros onto the
SONET/SDH interface.
The third case when a CEP de-packetizer MUST play out an SPE/VT
Unequipped indication towards the SONET/SDH interface is when the
circuit has been de-provisioned.
8. SONET/SDH Transport Timing
It is assumed that the distribution of SONET/SDH transport timing
information is addressed either through external mechanisms such as
Building Integrated Timing Supply (BITS), Stand Alone Synchronization
Equipment (SASE), Global Positioning System (GPS), or other such
methods, or is obtained through an adaptive timing recovery
mechanism.
Details about specific adaptive algorithms for recovery of SONET/SDH
transport timing are not considered to be within scope for this
document. The wander and jitter limits for networks based on the SDH
hierarchy are defined in [G.825] and for the SONET hierarchy in
[GR253]. The wander and jitter limits specified in these standards
must be maintained when CEP PWs are used.
Malis, et al. Standards Track [Page 16]

RFC 4842 SONET/SDH Circuit Emulation April 20079. SONET/SDH Pointer Management
A pointer management system is defined as part of the definition of
SONET/SDH. Details on SONET/SDH pointer management can be found in
[SONET], [GR253], [G.707], and [G.783]. If there is a frequency
offset between the frame rate of the transport overhead and that of
the SONET/SDH SPE, the alignment of the SPE will periodically slip
back or advance in time through positive or negative stuffing.
Similarly, if there is a frequency offset between the SPE rate and
the VT rate it carries, the alignment of the VT will periodically
slip back or advance in time through positive or negative stuffing
within the SPE.
The emulation of this aspect of SONET/SDH networks may be
accomplished using a variety of techniques including Explicit Pointer
Adjustment Relay (EPAR) and Adaptive Pointer Management (APM).
In any case, the handling of the SPE or VT data by the CEP packetizer
is the same.
During a negative pointer adjustment event, the CEP packetizer MUST
incorporate the H3 (or V3) byte from the SONET/SDH stream into the
CEP packet payload in order with the rest of the SPE (or VT). During
a positive pointer adjustment event, the CEP packetizer MUST strip
the stuff byte from the CEP packet payload.
When playing out a negative pointer adjustment event, the appropriate
byte of the CEP payload MUST be placed into the H3 (or V3) byte of
the SONET/SDH stream. When playing out a positive pointer
adjustment, the CEP de-packetizer MUST insert a stuff byte into the
appropriate position within the SONET/SDH stream.
The details regarding the use of the H3 (and V3) byte and stuff byte
during positive and negative pointer adjustments can be found in
[SONET], [GR253], and [G.707].
9.1. Explicit Pointer Adjustment Relay (EPAR)
CEP provides an OPTIONAL mechanism to explicitly relay pointer
adjustment events from one side of the PSN to the other. This
technique is referred to as Explicit Pointer Adjustment Relay (EPAR).
EPAR is only effective when both ends of the PW have access to a
common timing reference.
The following text only applies to CEP implementations that choose to
implement EPAR. Any CEP implementation that does not support EPAR
MUST set the N and P bits to 0.
Malis, et al. Standards Track [Page 17]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The pointer adjustment event MUST be transmitted in three consecutive
packets by the packetizer. The de-packetizer MUST play out the
pointer adjustment event when any one packet with N/P bit set is
received. The CEP de-packetizer MUST utilize the CEP sequence
numbers to ensure that SONET/SDH pointer adjustment events are not
played any more frequently than once per every three CEP packets
transmitted by the remote CEP packetizer.
The VT EPAR packetizer MUST relay pointer adjustment indications
received at the SPE level in addition to relaying VT pointer
adjustment indications. Because of the rate differences between VT
and SPE, the magnitude of a VT pointer adjustment is much larger than
that of an SPE adjustment. Therefore, the VT EPAR packetizer has to
convert multiple SPE pointer adjustments to fewer VT pointer
adjustment indications signaled over the PSN using the N and P CEP
header bits. A simple algorithm can be used for this purpose using
an accumulator (counter):
The accumulator value is reset to 0 when the circuit is in Loss of
Packet Synchronization (LOPS) state.
A positive pointer adjustment indication increases the accumulator
value by a fixed quota, while negative pointer adjustment
subtracts the same quota from the accumulator. A VT pointer
adjustment changes the accumulator value by 783 units (one STS-1
SPE size). An SPE pointer adjustment changes the accumulator
value by quota that depends on the VT emulation type. The quota
is 1/4 of the VT size as defined in Table 1, e.g., 26 bytes for
VT1.5 emulation and 35 bytes for VT2 emulation.
When the accumulator value is larger than or equal to 783 units, a
positive pointer adjustment is signaled towards the PSN using the
CEP header P bit and 783 units are subtracted from the
accumulator.
When the accumulator value is smaller than or equal to minus 783
units, a negative pointer adjustment is signaled towards the PSN
using the CEP header N bit and 783 units are added to the
accumulator.
The same algorithm is applicable for SDH Virtual Container carried in
VC-4, i.e., positive VC-4 pointer adjustment adds 35 units to a VC-12
accumulator, while positive VC-12 pointer adjustment adds 783 units
to the accumulator.
Malis, et al. Standards Track [Page 18]

RFC 4842 SONET/SDH Circuit Emulation April 2007
If both N and P bits are set, then a Loss of Pointer event has been
detected at the PW ingress, making the pointer invalid. The de-
packetizer MUST play out an AIS-P/V indication and SHOULD set the
pointer value to all-ones value.
9.2. Adaptive Pointer Management (APM)
Another OPTIONAL method that may be used to emulate SONET/SDH pointer
management is Adaptive Pointer Management (APM). In basic terms, APM
uses information about the depth of the CEP jitter buffers to
introduce pointer adjustments in the reassembled SONET/SDH SPE.
Details about specific APM algorithms are not considered to be within
scope for this document.
10. CEP Performance Monitors
SONET/SDH as defined in [SONET], [GR253], [G.707], and [G.784]
includes a definition of several counters used to monitor the
performance of SONET/SDH services. These counters are referred to as
Performance Monitors.
In order for CEP to be utilized by traditional SONET/SDH network
operators, CEP SHOULD provide similar functionality. The following
sections describe a number of counters that are collectively referred
to as CEP Performance Monitors.
10.1. Near-End Performance Monitors
These performance monitors are maintained by the CEP de-packetizer
during reassembly of the SONET/SDH stream.
The performance monitors are based on two types of defects.
Type 1: missing or dropped packet.
Type 2: buffer underrun, buffer overrun, LOPS, missing packets
above predefined configurable threshold.
The specific performance monitors defined for CEP are as follows:
ES-CEP - CEP Errored Seconds
SES-CEP - CEP Severely Errored Seconds
UAS-CEP - CEP Unavailable Seconds
Malis, et al. Standards Track [Page 19]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Each second that contains at least one type 1 defect SHALL be
declared as ES-CEP. Each second that contains at least one type 2
defect SHALL be declared as SES-CEP.
UAS-CEP SHALL be declared after configurable number of consecutive
SES-CEP, and cleared after a configurable number of consecutive
seconds without SES-CEP. Default value for each is 10 seconds.
Once unavailability is declared, ES and SES counts SHALL be inhibited
up to the point where the unavailability was started. Once
unavailability is removed, ES and SES that occurred along the
clearing period SHALL be added to the ES and SES counts.
CEP-NE failure is declared after 2.5 +/- 0.5 seconds of CEP-NE type 2
defect, and cleared after 10 seconds free of CEP-NE defect state.
Sending notification to the OS for CEP-NE failure is local policy.
10.2. Far-End Performance Monitors
Far-end performance monitors provide insight into the CEP de-
packetizer at the far end of the PSN.
Far-end statistics are based on the CEP-RDI indication carried in the
CEP header R bit. CEP-FE defect is declared when CEP-RDI is set in
the incoming CEP packets.
CEP-FE failure is declared after 2.5 +/- 0.5 seconds of CEP-FE
defect, and cleared after 10 seconds free of CEP-FE defect state.
Sending notification to the OS for CEP-FE failure is local policy.
11. Payload Compression Options
In addition to pure emulation, CEP also offers a number of options
for reducing the total bandwidth utilized by the emulated circuit.
These options fall into two categories: Dynamic Bandwidth Allocation
(DBA) and Service-Specific Payload Formats.
DBA suppresses transmission of the CEP payload altogether under
certain circumstances such as AIS-P/V and SPE/VT Unequipped. The use
of DBA is dependent on network architectures, e.g., support of Tandem
Connection Monitoring, test signals (TEST-P) [SONET], or Supervisory
Unequipped [G.806] signals.
Service-Specific Payload Formats reduce bandwidth by suppressing
transmission of portions of the SPE based on specific knowledge of
the SPE payload.
Malis, et al. Standards Track [Page 20]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Details on these payload compression options are described in the
following subsections.
11.1. Dynamic Bandwidth Allocation
Dynamic Bandwidth Allocation (DBA) is an OPTIONAL mechanism for
suppressing the transmission of the SPE (or VT) fragment when one of
two trigger conditions are met, AIS-P/V or SPE/VT Unequipped.
Implementations that support DBA MUST include a mechanism for
disabling DBA on a channel-by-channel basis to allow for
interoperability with implementations that do not support DBA.
When a DBA trigger is recognized at PW ingress, the CEP payload will
be suppressed. The CEP Length field MUST be set to the CEP header
length plus the RTP header length if used, and padding bytes SHOULD
be added if the intervening packet network has a minimum packet size
that is larger than the payload-suppressed DBA packet.
Other than the suppression of the CEP payload, the CEP behavior
during DBA should be equivalent to normal CEP behavior. In
particular, the packet transmission rate during DBA should be
equivalent to the normal packet transmission rate.
11.2. Service-Specific Payload Formats
In addition to the standard payload encapsulations for SPE and VT
transport, several OPTIONAL payload formats have been defined to
provide varying amounts of payload compression depending on the type
and amount of user traffic present within the SPE. These are
described below.
11.2.1. Fractional STS-1 (VC-3) Encapsulation
Fractional STS-1 (VC-3) encapsulation carries only a selected set of
VTs within an STS-1 container. This mode is applicable for STS-1
with POH signal label byte C2=2 (VT-structured SPE) and for C2=3
(Locked VT mode).
Implementations of fractional STS-1 (VC-3) encapsulation MUST support
payload length of one SPE and MAY support payload length of 1/3 SPE.
The mapping of VTs into an STS-1 container is described in Section3.2.4 of [GR253], and the mapping into VC-3 is defined in Section7.2.4 in [G.707]. The CEP packetizer removes all fixed column bytes
(columns 30 and 59) and all bytes belonging to the removed VTs. Only
Malis, et al. Standards Track [Page 21]

RFC 4842 SONET/SDH Circuit Emulation April 2007
with VT1.5 tributaries is all ones, while that of an STS-1 fully
occupied with VT2 (E1) tributaries has the binary value
0111011101110111011101110111.
11.2.1.2. B3 Compensation
Fractional STS-1 encapsulation can be implemented in Line Terminating
Equipment (LTE) or in Path Terminating Equipment (PTE). PTE
implementations terminate the path layer at the ingress PE and
generate a new path layer at the egress PE.
LTE implementations do not terminate the path layer, and therefore
need to keep the content and integrity of the POH bytes across the
PSN. In LTE implementations, special care must be taken to maintain
the B3 bit-wise parity POH byte. The B3 compensation algorithm is
defined below.
Since the BIP-8 value in a given frame reflects the parity check over
the previous frame, the changes made to BIP-8 bits in the previous
frame shall also be considered in the compensation of BIP-8 in the
current frame. Therefore, the following equation shall be used for
compensation of the individual bits of the BIP-8:
B3[i]'(t) = B3[i](t-1) || B3[i]'(t-1) || B3[i](t) || B*3[i](t-1)
Where:
B3[i] = the existing B3[i] value in the incoming signal
B3[i]' = the new (compensated) B3[i] value
B3*[i] = the B3[i] value of the unequipped VTs in the
incoming signal
|| = exclusive OR operator
t = the time of the current frame
t-1 = the time of the previous frame
The egress PE MUST reconstruct the unequipped VTs and modify the B3
parity value in the same manner to accommodate the additional VTs
added. In this way, the end-to-end BIP is preserved.
11.2.1.3. Actual Payload Size
The actual CEP payload size depends on the number of virtual
tributaries carried within the fractional SPE. The contributions of
each tributary to the fractional STS-1 payload size as well as the
path overhead contribution are described below.
Each VT1.5 contributes 27 bytes
Malis, et al. Standards Track [Page 24]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Each VT2 contributes 36 bytes
Each VT3 contributes 54 bytes
Each VT6 contributes 108 bytes
STS-1 POH contributes 9 bytes
For example, a fractional STS-1 carrying 7 VT2 circuit in full-SPE
encapsulation would have an actual size of 261=36*7+9 bytes. Divide
by 3 to calculate the third-SPE encapsulation actual payload sizes.
11.2.2. Asynchronous T3/E3 STS-1 (VC-3) Encapsulation
Asynchronous T3/E3 STS-1 (VC-3) encapsulation is applicable for
signals with POH signal label byte C2=4, carrying asynchronously
mapped T3 or E3 signals.
Implementations of asynchronous T3/E3 STS-1 (VC-3) encapsulation MUST
support payload length of one SPE and MAY support payload length of
1/3 SPE.
11.2.2.1. T3 Payload Compression
A T3 is encapsulated asynchronously into an STS-1 SPE using the
format defined in Section 3.4.2.1 of [GR253]. The STS-1 SPE is then
encapsulated as defined in this document.
Optionally, the STS-1 SPE can be compressed by removing the fixed
columns leaving only data columns. STS-1 columns are numbered 1 to
87, starting from the POH column numbered 1. The following columns
have fixed values and are removed: 2, 3, 30, 31, 59, and 60.
Bandwidth saving is approximately 7% (6 columns out of 87). The B3
parity byte need not be modified as the parity of the fixed columns
amounts to 0. The actual payload size for full-SPE encapsulation
would be 729 bytes and 243 bytes for third-SPE encapsulation.
A T3 is encapsulated asynchronously into a VC-3 container as
described in Section 10.1.2.1 of [G.707]. VC-3 container has only 85
data columns, which is identical to the STS-1 container with the two
fixed stuff columns 30 and 59 removed. Other than that, the mapping
is identical.
Malis, et al. Standards Track [Page 25]

RFC 4842 SONET/SDH Circuit Emulation April 200711.2.2.2. E3 Payload Compression
An E3 is encapsulated asynchronously into a VC-3 SPE using the format
defined in Section 10.1.2.2 of [G.707]. The VC-3 SPE is then
encapsulated as defined in this document.
Optionally, the VC-3 SPE can be compressed by removing the fixed
columns leaving only data columns. VC-3 columns are numbered 1 to 85
(and not 87), starting from the POH column numbered 1. The following
columns have fixed values and are removed: 2, 6, 10, 14, 18, 19, 23,
27, 31, 35, 39, 44, 48, 52, 56, 60, 61, 65, 69, 73, 77, and 81.
Bandwidth saving is approximately 28% (24 columns out of 85). The B3
parity byte need not be modified as the parity of the fixed columns
amounts to 0. The actual payload size for full-SPE encapsulation
would be 567 bytes and 189 bytes for third-SPE encapsulation.
11.2.3. Fractional VC-4 Encapsulation
SDH defines a mapping of VC-11, VC-12, VC-2, and VC-3 directly into
VC-4. This mapping does not have an equivalent within the SONET
hierarchy. The VC-4 mapping is common in SDH implementations.
VC-4 <--x3-- TUG-3 <--------x1-------- TU-3 <-- VC-3 <---- E3/T3
|
+--x7-- TUG-2 <--x1-- TU-2 <-- VC-2 <---- DS2
|
+----x3---- TU-12 <-- VC-12<---- E1
|
+----x4---- TU-11 <-- VC-11<---- T1
Figure 8: Mapping of VCs into VC-4
Figure 8 describes the mapping options of VCs into VC-4. A VC-4
contains three TUG-3s. Each TUG-3 is composed of either a single
TU-3 or 7 TUG-2s. A TU-3 contains a single VC-3. A TUG-2 contains
either 4 VC-11s (T1s), 3 VC-12s (E1s), or one VC-2. Therefore, a
VC-4 may contain 3 VC-3s, 1 VC-3 and 42 VC-12s, 63 VC-12s, etc.
Fractional VC-4 encapsulation carries only a selected set of VCs
within a VC-4 container. This mode is applicable for VC-4 with POH
signal label byte C2=2 (TUG structure) and for C2=3 (Locked TU-n).
The mapping of VCs into a VC-4 container is described in Section 7.2
of [G.707]. The CEP packetizer removes all fixed column bytes and
all bytes that belong to the removed VCs. Only VC-4 POH bytes and
bytes that belong to the selected VCs are carried within the payload.
The CEP de-packetizer adds the fixed stuff bytes and generates
unequipped VC data replacing the removed VC bytes.
Malis, et al. Standards Track [Page 26]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The fractional VC-4 encapsulation can optionally carry a bit mask
that specifies which VCs are carried within the VC-4 payload and
which VCs have been removed. This optional bit mask attribute allows
the ingress circuit emulation node to remove unequipped VCs on the
fly, providing the egress circuit emulation node enough information
for reconstructing the VCs in the right order. The use of bit mask
enables on-the-fly compression, whereby only equipped VCs (carrying
actual data) are sent.
VC-3 carrying asynchronous T3/E3 signals within the VC-4 container
can optionally be compressed by removing the fixed column bytes as
described in Section 11.2.2, providing additional bandwidth saving.
Implementations of fractional VC-4 encapsulation MUST support payload
length of 1/3 SPE and MAY support payload lengths of 4/9, 5/9, 6/9,
7/9, 8/9, and full SPE. The actual payload size of fractional VC-4
encapsulation depends on the number of VCs carried within the
payload.
11.2.3.1. Fractional VC-4 Mapping
[G.707] defines the mapping of TUG-3 to a VC-4 in Section 7.2.1.
Each TUG-3 includes 86 columns. TUG-3#1, TUG-3#2, and TUG-3#3 are
byte multiplexed, starting from column 4. Column 1 is the VC-4 POH,
while columns 2 and 3 are fixed and therefore removed in the
fractional VC-4 encapsulation.
The mapping of TU-3 into TUG-3 is defined in Section 7.2.2 of
[G.707]. The TU-3 consists of the VC-3 with a 9-byte VC-3 POH and
the TU-3 pointer. The first column of the 9-row-by-86-column TUG-3
is allocated to the TU-3 pointer (bytes H1, H2, H3) and fixed stuff.
The phase of the VC-3 with respect to the TUG-3 is indicated by the
TU-3 pointer.
The mapping of TUG-2 into TUG-3 is defined in Section 7.2.3 of
[G.707]. The first two columns of the TUG-3 are fixed and therefore
removed in the fractional VC-4 encapsulation. The 7 TUG-2s, each 12
columns wide, are byte multiplexed starting from column 3 of the
TUG-3. This is the equivalent of multiplexing 7 VTGs within STS-1
container in SONET terminology, except for the location of the fixed
columns.
The static fractional VC-4 mapping assumes that both the ingress and
egress nodes are preconfigured with the set of equipped VCs carried
within the fractional VC-4 encapsulation. The ingress emulation edge
removes the fixed columns as well as columns of the VCs agreed upon
by the two edges, and updates the B3 VC-4 byte. The egress side adds
the fixed columns and the unequipped VCs and updates B3.
Malis, et al. Standards Track [Page 27]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The VC-4 EBM has the following format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|T|TUG2#7 |TUG2#6 |TUG2#5 |TUG2#4 |TUG2#3 |TUG2#2 |TUG2#1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Equipped Bit Mask (EBM) for Fractional VC-4
The 30 bits of the EBM are divided into 2 bits that control the VC-3
within the TUG-3 and 7 groups of 4 bits, each corresponding to a
different TUG-2 within the TUG-3 container.
For a TUG-3 containing TUG-2, the first two A and T bits MUST be set
to 0. The TUG-2 bits indicate whether the VCs within the TUG-2 are
equipped. All 4 bits are used to indicate whether VC-11 (T1)
tributaries are carried within a TUG-2. The first 3 bits read right
to left are used to indicate whether VC-12 (E1) tributaries are
carried within a TUG-2. The first bit is used to indicate that a
VC-2 is carried within a TUG-2. The VCs within the TUG-2 are
numbered from right to left, starting from the first VC as the first
bit on the right. For example, 28 bits of the EBM of a fully
occupied TUG-3 with VC-11 tributaries are all ones, while that of a
TUG-3 fully occupied with VC-12 tributaries has the binary value
000111011101110111011101110111.
For a TUG-3 containing VC-3, all TUG-2 bits MUST be set to 0. The A
and T bits are defined as follows:
T: TUG-3 carried bit. If set to 1, the VC-3 payload is carried
within the TUG-3 container. If set to 0, all the TUG-3 columns are
not carried within the fractional VC-4 encapsulation. The TUG-3
columns are removed either because the VC-3 is unequipped or in AIS
mode.
A: VC-3 AIS bit. The A bit MUST be set to 0 when the T bit is 1
(i.e., when the TUG-3 columns are carried within the fractional VC-4
encapsulation). The A bit indicate the reason for removal of the
entire TUG-3 columns. If set to 0, the TUG-3 columns were removed
because the VC-3 is unequipped. If set to 1, the TUG-3 columns were
removed because the VC-3 is in AIS mode.
11.2.3.3. B3 Compensation
Fractional VC-4 encapsulation can be implemented in Line Terminating
Equipment (LTE) or in Path Terminating Equipment (PTE). PTE
implementations terminate the path layer at the ingress PE and
Malis, et al. Standards Track [Page 29]

RFC 4842 SONET/SDH Circuit Emulation April 2007
generate a new path layer at the egress PE. LTE implementations do
not terminate the path layer, and therefore need to keep the content
and integrity of the POH bytes across the PSN. In LTE
implementations, special care must be taken to maintain the B3 bit-
wise parity POH byte. The same procedures for B3 compensation as
described in Section 11.2.1.2 for fractional STS-1 encapsulation are
used.
11.2.3.4. Actual Payload Sizes
The actual CEP payload size depends on the number of virtual
tributaries carried within the fractional SPE. The contributions of
each tributary to the fractional VC-4 payload length as well as the
path overhead contribution are described below.
Each VC-11 contributes 27 bytes
Each VC-12 contributes 36 bytes
Each VC-2 contributes 108 bytes
Each VC-3(T3) contributes 738 bytes
Each VC-3(E3) contributes 576 bytes
Each VC-3(uncompressed) contributes 774 bytes
VC-4 POH contributes 9 bytes
The VC-3 contribution includes the AU-3 pointer. For example, the
payload size of a fractional VC-4 configured to third-SPE
encapsulation that carries a single compressed T3 VC-3 and 6 VC-12s
would be: 321=(9 + 6*36 + 738) / 3 bytes payload per each packet.
12. Signaling of CEP Pseudowires
[PWE3-CONTROL] specifies the use of the MPLS Label Distribution
Protocol, LDP, as a protocol for setting up and maintaining
pseudowires. In particular, it provides a way to bind a de-
multiplexer field value to a pseudo-wire, specifying procedures for
reporting pseudowire status changes and for releasing the bindings.
[PWE3-CONTROL] assumes that the pseudowire de-multiplexer field is an
MPLS label; however, the PSN tunnel itself can be either an IP or
MPLS PSN.
The use of LDP for setting up and maintaining CEP pseudowires is
OPTIONAL. This section describes the use of the CEP-specific fields
and error codes.
Malis, et al. Standards Track [Page 30]

RFC 4842 SONET/SDH Circuit Emulation April 2007
The PW Type field in PWid Forwarding Equivalence Class (FEC) and PW
generalized ID FEC elements MUST be set to SONET/SDH Circuit
Emulation over Packet (CEP) [PWE3-IANA].
The control word is REQUIRED for CEP pseudowires. Therefore, the C
bit in PWid FEC and PW generalized ID FEC elements MUST be set. If
the C bit is not set, the pseudowire MUST not be established and a
Label Release MUST be sent with an Illegal C bit status code
[PWE3-IANA].
The PWid FEC and PW generalized ID FEC elements can include one or
more Interface Parameters fields. The Interface Parameters fields
are used to validate that the two ends of the pseudowire have the
necessary capabilities to interoperate with each other. The CEP-
specific Interface Parameters fields are the CEP/TDM Payload Bytes,
the CEP/TDM Bit Rate, and the CEP Options parameters.
12.1. CEP/TDM Payload Bytes
This parameter MUST contain the expected CEP payload size in bytes.
The payload size does not include network headers, CEP header or
padding. If payload compression is used, the CEP/TDM Payload Bytes
parameter MUST be set to the uncompressed payload size as if payload
compression was disabled. In particular, when Fractional SPE (STS-1/
VC-3 or VC-4) payload compression is used, the Payload Bytes
parameter MUST be set to the payload size before removal of the
unequipped VT containers and fixed value columns. Therefore, when
fractional SPE mode is used, the actual (i.e., on the wire) packet
length would normally be less than advertised, and in dynamic
fractional SPE, even change while the connection is active.
Similarly, when DBA payload compression is used, the CEP/TDM Payload
Bytes parameter MUST be set to the payload size prior to compression.
The CEP/TDM Payload Bytes parameter is OPTIONAL. Default payload
sizes are assumed if this parameter is not included as part of the
Interface Parameters fields. The default payload size for VT is a
single super frame. The default payload size for SPE is 783 bytes.
A PE that receives a label-mapping request with request for a CEP/TDM
Payload Bytes value that is not locally supported MUST return CEP/TDM
misconfiguration status error code [PWE3-IANA], and the pseudowire
MUST not be established.
12.2. CEP/TDM Bit Rate
The CEP/TDM Bit Rate parameter MUST be set to the data rate in 64-
Kbps units of the CEP payload. If payload compression is used, the
CEP/TDM Bit Rate parameter MUST be set to the uncompressed payload
Malis, et al. Standards Track [Page 31]

RFC 4842 SONET/SDH Circuit Emulation April 2007
EBM: When set, indicates that the PE sending the label-mapping
request is configured to send packets with EBM extension header.
CEP Type: indicates the CEP connection type:
0x0 SPE mode (STS-1/STS-Mc)
0x1 VT mode (VT1.5/VT2/VT3/VT6)
0x2 Fractional SPE (STS-1/VC-3/VC-4)
Async Type: indicates the Async E3/T3 bandwidth reduction
configuration. Relevant only when CEP type is set to fractional
SPE, and fractional SPE is expected to carry Asynchronous T3/E3
payload:
T3: When set, indicates that the PE sending the label-mapping
request is configured to send Fractional SPE packets with T3
bandwidth reduction.
E3: When set, indicates that the PE sending the label-mapping
request is configured to send Fractional SPE packets with E3
bandwidth reduction.
Reserved field: MUST be set to 0 by the PE sending the label-mapping
request and ignored by the receiver.
A PE that does not support one of the CEP options set in the label-
mapping request MUST send a label-release message with status code of
CEP/TDM misconfiguration [PWE3-IANA], report to the operator, and
wait for a new consistent label-mapping. A PE MUST send a new label-
mapping request once it is reconfigured or when it receives a label-
mapping request from its peer with consistent configuration.
A pseudowire can be configured asymmetrically. One PE can be
configured to use bandwidth reduction modes, while the other PE can
be configured to send the entire circuit unmodified. A PE can
compare the CEP Options settings received in the label-mapping
request with its own configuration and detect an asymmetric
pseudowire configuration. A PE that identifies an asymmetric
configuration MAY report it to the operator.
Malis, et al. Standards Track [Page 33]

RFC 4842 SONET/SDH Circuit Emulation April 200713. Congestion Control
The PSN carrying the CEP PW may be subject to congestion. Congestion
considerations for PWs are described in Section 6.5 of [PWE3-ARCH].
CEP PWs represent inelastic constant bit rate (CBR) flows and cannot
respond to congestion in a TCP-friendly manner prescribed by [CONG].
CEP PWs SHOULD be carried across traffic-engineered PSNs that provide
either bandwidth reservation and admission control or forwarding
prioritization and boundary traffic conditioning mechanisms.
Intserv-enabled domains [INTSERV] supporting Guaranteed Service [GS]
and Diffserv-enabled domains [DIFFSERV] supporting Expedited
Forwarding [EF] provide examples of such PSNs. It is expected that
PWs emulating high-rate SONET STS-Nc or SDH virtual circuits will be
tunneled over traffic-engineered MPLS PSN.
CEP PWs SHOULD monitor packet loss in order to detect "severe
congestion". If such a condition is detected, a CEP PW SHOULD shut
down bi-directionally. This specification does not define the exact
criteria for detecting "severe congestion" using the CEP packet loss
rate and the consequent restart criteria after a suitable delay.
This is left for further study.
If the CEP PW has been set up using the PWE3 control protocol
[PWE3-CONTROL], the regular PW teardown procedures SHOULD be used
upon detection of "severe congestion".
The SONET/SDH services emulated by CEP PWs have high availability
objectives that MUST be taken into account when deciding on temporary
shutdown of CEP PWs. CEP performance monitoring provides entry and
exit criteria for the CEP PW unavailable state (UAS-CEP). Detection
of "severe congestion" MAY be based on unavailability criteria of the
CEP PW.
14. Security Considerations
The CEP encapsulation is subject to all of the general security
considerations discussed in [PWE3-ARCH]. In addition, this document
specifies only encapsulations, and not the protocols used to carry
the encapsulated packets across the PSN. Each such protocol may have
its own set of security issues, but those issues are not affected by
the encapsulations specified herein. Note that the security of the
transported CEP service will only be as good as the security of the
PSN. This level of security may be less rigorous than that available
from a native TDM service due to the inherent differences between
circuit-switched and packet-switched public networks.
Malis, et al. Standards Track [Page 34]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Although CEP MAY employ an RTP header when explicit transfer of
timing information is required, SRTP [RFC3711] mechanisms are not a
substitute for securing the PW and underlying MPLS network.
15. IANA Considerations
IANA considerations for pseudowires are covered in [PWE3-IANA]. CEP
does not introduce additional requirements from IANA.
16. Acknowledgments
The authors would like to thank the members of the PWE3 Working Group
for their assistance on this document. We thank Sasha Vainshtein,
Deborah Brungard, Juergen Heiles, and Nick Weeds for their review and
valuable feedback.
17. Co-Authors
The individuals listed below are co-authors of this document. Tom
Johnson from Litchfield Communications was the editor of this
document from the pre-WG versions of the SONET SPE work through
version 01 of this document.
Craig White Level3 Communications
Ed Hallman Litchfield Communications
Jeremy Brayley Laurel Networks
Jim Boyle Juniper Networks
John Shirron Laurel Networks
Luca Martini Cisco Systems
Marlene Drost Litchfield Communications
Steve Vogelsang Laurel Networks
Tom Johnson Litchfield Communications
Ken Hsu Tellabs
Malis, et al. Standards Track [Page 35]

RFC 4842 SONET/SDH Circuit Emulation April 2007Appendix A. SONET/SDH Rates and Formats
For simplicity, the discussion in this section uses SONET
terminology, but it applies equally to SDH as well. SDH-equivalent
terminology is shown in the tables.
The basic SONET modular signal is the synchronous transport signal-
level 1 (STS-1). A number of STS-1s may be multiplexed into higher-
level signals denoted as STS-N, with N synchronous payload envelopes
(SPEs). The optical counterpart of the STS-N is the Optical Carrier-
level N, or OC-N. Table 4 lists standard SONET line rates discussed
in this document.
+-------------+--------+---------+----------+-----------+-----------+
| OC Level | OC-1 | OC-3 | OC-12 | OC-48 | OC-192 |
+-------------+--------+---------+----------+-----------+-----------+
| SDH Term | - | STM-1 | STM-4 | STM-16 | STM-64 |
| Line | 51.840 | 155.520 | 622.080 | 2,488.320 | 9,953.280 |
| Rate(Mb/s) | | | | | |
+-------------+--------+---------+----------+-----------+-----------+
Table 4: Standard SONET Line Rates
Each SONET frame is 125us and consists of nine rows. An STS-N frame
has nine rows and N*90 columns. Of the N*90 columns, the first N*3
columns are transport overhead and the other N*87 columns are SPEs.
A number of STS-1s may also be linked together to form a super-rate
signal with only one SPE. The optical super-rate signal is denoted
as OC-Nc, which has a higher payload capacity than OC-N.
The first 9-byte column of each SPE is the path overhead (POH) and
the remaining columns form the payload capacity with fixed stuff
(STS-Nc only). The fixed stuff, which is purely overhead, is N/3-1
columns for STS-Nc. Thus, STS-1 and STS-3c do not have any fixed
stuff, STS-12c has three columns of fixed stuff, and so on.
The POH of an STS-1 or STS-Nc is always 9 bytes in nine rows. The
payload capacity of an STS-1 is 86 columns (774 bytes) per frame.
The payload capacity of an STS-Nc is (N*87)-(N/3) columns per frame.
Thus, the payload capacity of an STS-3c is (3*87 - 1)*9 = 2,340 bytes
per frame. As another example, the payload capacity of an STS-192c
is 149,760 bytes, which is 64 times the capacity of an STS-3c.
There are 8,000 SONET frames per second. Therefore, the SPE size,
(POH plus payload capacity) of an STS-1 is 783*8*8,000 = 50.112 Mb/s.
The SPE size of a concatenated STS-3c is 2,349 bytes per frame or
Malis, et al. Standards Track [Page 36]

RFC 4842 SONET/SDH Circuit Emulation April 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
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Malis, et al. Standards Track [Page 43]